Apoptosis is a form of programmed cell death which occurs through the activation of cell-intrinsic suicide machinery. The biochemical machinery responsible for apoptosis is expressed in most, if not all, cells. Apoptosis is primarily a physiologic process necessary to remove individual cells that are no longer needed or that function abnormally. Apoptosis is a regulated event dependent upon active metabolism and protein synthesis by the dying cell.
Apoptosis plays a major role during development and homeostasis. Apoptosis can be triggered in a variety of cell types by the deprivation of growth factors, which appear to repress an active suicide response. Apoptosis is particularly important for the physiology of the immune system. Apoptosis is the mode of death of centroblasts with low affinity for antigen within germinal centers, cells killed by specific cytotoxic T lymphocytes or natural killer cells, as well as thymocytes bearing high-affinity T-cell receptors for self antigens that are clonally deleted during thymus development (negative selection).
The morphological and biochemical characteristics of cells dying by apoptosis differ markedly from those of cells dying by necrosis. During apoptosis, cells decrease in size and round up. The nuclear chromatin undergoes condensation and fragmentation. Cell death is preceded by DNA fragmentation. The DNA of apoptotic cells is nonrandomly degraded by endogenous calcium and magnesium-dependent endonuclease(s) inhibited by zinc ions. This enzyme(s) gives fragments of approx. 200 base pairs (bp) or multiples of 200 bp by cutting the linker DNA running between nucleosomes. Thus DNA appears to be one of the most important targets of the process that leads to cell suicide. The apoptotic cell then breaks apart into many plasma membrane-bound vesicles called "apoptotic bodies," which contain fragments of condensed chromatin and morphologically intact organelles such as mitochondria. Apoptotic cells and bodies are rapidly phagocytosed, thereby protecting surrounding tissues from injury. The rapid and efficient clearance of apoptotic cells makes apoptosis extremely difficult to detect in tissue sections.
In contrast, necrosis is associated with rapid metabolic collapse that leads to cell swelling, early loss of plasma membrane integrity, and ultimate cell rupture. Cytosolic contents leach from the necrotic cell causing injury and inflammation to surrounding tissue.
Recent studies show that multiple cytotoxic stimuli well known to cause necrosis can lead to apoptosis instead when cells are exposed to the same noxious agents at lower concentrations. This insight has led to an interest in the role of apoptosis in the pathogenesis of renal diseases that result primarily from injury to renal tubular epithelial cells. These diseases include acute and chronic renal failure from exposure of the kidney to ischemia or to cytotoxic agents. There is also an interest in the role of apoptosis in Alzheimers and other neurological diseases.
Several genes associated with apoptosis have been identified. Examples include the p53 gene, which is involved in osteosarcoma and adrenocortical, breast and brain cancers;
The gene cloned and sequenced as described herein, ING1 (formerly called p33.sup.IG1), represents a new gene which is expressed in normal mammary epithelial cells, but expressed only at lower levels in several cancerous mammary epithelial cell lines and is not expressed in many primary brain tumors.